Sodium Based Battery Technologies

Sodium based batteries have a high energy density, long cycle life and can operate in harsh environments such as temperatures of -40°C to +60°C. For these reasons they can be found in application in energy grid storage, such as storing energy from intermittent energy sources such as wind- and solar-power.

Unlike many batteries, sodium based batteries consist of a solid or solid and molten electrolyte with liquid sodium acting as the negative electrode. These batteries are usually constructed in a cylindrical form, encased in a container which acts as the positive electrode. The chemistry is quite simple with no side reactions and roundtrip efficiency (charge/discharge) of up to 85%.

Sodium – Nickel Chloride Technology

The cathode is Nickel Chloride (Ni CL2) while the anode is made of Sodium (Na). The electrolyte is made up of tetrachloralluminate of sodium (such as NaAlCL4), and is liquid at the operating temperature of the cells (and battery) in between 270 °C and 350°C. When the battery is being charged the Sodium atoms in the cathode become ions and migrate through the ceramic electrolyte (beta Al). Available free electrons could flow as current to an external load. This process is reversed during discharge.The main advantages of Na- NiCl2 batteries, compared to other advanced batteries, are:

Commercialized since the middle of the 1990's, Sodium Nickel Chloride batteries have found application in EV (electric vehicle) cars and HEV (hybrid electric vehicle) buses, trucks, vans. The implementation of Sodium Nickel Chloride batteries in the stationary field is in its starting phase. Demonstration systems combined with distributed renewable generators (large PV plants and micro wind turbine) as well as for grid support with voltages up to 600V have been designed and are now in field test phase.

No toxic or dangerous materials are used during manufacture of Sodium – Nickel Chloride batteries. The battery is fully recyclable within existing industries for the production of stainless steel and road paving.

Sodium – Sulfur technology

This battery has a solid electrolyte membrane between the anode and cathode, compared to liquid metal batteries where the anode, the cathode, and also the membrane are liquids. The cell is usually made in a tall cylindrical configuration. The entire cell is enclosed by a steel casing that is protected, usually by chromium and molybdenum, from corrosion on the inside. This outside container serves as the positive electrode, while the liquid sodium serves as the negative electrode. The container is sealed at the top with an airtight alumina lid. An essential part of the cell is the presence of a BASE (Beta-alumina solid electrolyte) membrane, which selectively conducts Na+. The cell becomes more economical with increasing size. In commercial applications the cells are arranged in blocks for better conservation of heat and are encased in a vacuum-insulated box.

During the discharge phase, molten elemental sodium at the core serves as the anode, meaning that the Na donates electrons to the external circuit. The sodium is separated by a beta-alumina solid electrolyte (BASE) cylinder from a container of molten sulfur, which is fabricated from an inert metal serving as the cathode. The sulfur is absorbed in a carbon sponge. BASE is a good conductor of sodium ions, but a poor conductor of electrons, and thus avoids self-discharge.

As the cell discharges, the sodium level drops. During the charging phase the reverse process takes place. Once running, the heat produced by charging and discharging cycles is sufficient to maintain operating temperatures and usually no external source is required.

The NaS battery is used in pilot projects to develop a durable utility power storage device due to its efficiency of 70% or better and a lifetime of over 1,500 cycles.